[R] why NA coefficients
array chip
arrayprofile at yahoo.com
Tue Nov 8 01:33:14 CET 2011
Hi, I am trying to run ANOVA with an interaction term on 2 factors (treat has 7 levels, group has 2 levels). I found the coefficient for the last interaction term is always 0, see attached dataset and the code below:
> test<-read.table("test.txt",sep='\t',header=T,row.names=NULL)
> lm(y~factor(treat)*factor(group),test)
Call:
lm(formula = y ~ factor(treat) * factor(group), data = test)
Coefficients:
(Intercept) factor(treat)2 factor(treat)3
0.429244 0.499982 0.352971
factor(treat)4 factor(treat)5 factor(treat)6
-0.204752 0.142042 0.044155
factor(treat)7 factor(group)2 factor(treat)2:factor(group)2
-0.007775 -0.337907 -0.208734
factor(treat)3:factor(group)2 factor(treat)4:factor(group)2 factor(treat)5:factor(group)2
-0.195138 0.800029 0.227514
factor(treat)6:factor(group)2 factor(treat)7:factor(group)2
0.331548 NA
I guess this is due to model matrix being singular or collinearity among the matrix columns? But I can't figure out how the matrix is singular in this case? Can someone show me why this is the case?
Thanks
John
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